1. Field of the Invention
This invention relates to a monolithic liquid crystal display device of the active matrix type which has a thin film transistor drive circuit in a liquid crystal cell and more particularly, to a passivation structure of a thin film transistor in the liquid crystal display device.
2. Description of the Prior Art
A known liquid crystal display device is first described. FIG. 21 schematically shows a known liquid crystal display device which includes a horizontal drive 4 and a vertical driver 5, each formed with a thin film transistor (TFT), outside a liquid cell unit U. The liquid cell unit U alone is so arranged that a liquid crystal layer (not shown) is sealed in a spacer 16 sandwiched between an upper substrate 14 and a lower substrate 15 which are opposed to each other. In other words, the lower substrate 15 serving as a substrate for the TFT""s is made larger in size than the upper substrate 14 to define a marginal substrate portion on which the peripheral drivers 4 and 5 are mounted.
In this arrangement where the drive circuits 4, 5 having TFT are mounted outside the liquid crystal cell unit U, it is necessary to form a protective layer made of SiN on the TFT portion of the drive circuits so as to protect the TFT portion from moisture and movable ions such as sodium ions.
On the other hand, within the liquid crystal cell, there are used polyimide resins as an orienting agent, so that it is essential to use a TFT transistor protective insulating film, made of a silicon dioxide compound, on picture elements.
When a SiN-based insulating film is used as the protective film instead of the SiO2-based compound within the liquid crystal cell, the adhesiveness and uniformity of the polyimide orientation film are disadvantageously impeded. As a result, the orientation of the liquid crystal is in turn impeded. In addition, when the SiN-based protective layer is formed on the upper portion of the TFT transistor, the film stress left in the SiN layer will undesirably change the transistor characteristics, e.g. the VTH characteristic is shifted.
Japanese Patent Publication No. 2-61032 proposes a liquid crystal display device which has drive circuits built in a liquid crystal cell. This liquid crystal display device is not defined with respect to the type of material for the passivation film to protect thin film transistors therewith. Thus, the problem involved in the SiN insulating film is still left.
In the monolithic type of liquid crystal display device set forth hereinabove, thin film transistors are constituted of polysilicon and contain grain boundaries. At the grain boundaries, trapping levels are so high that carriers are trapped. By the trapping, the boundaries are charged to form a barrier potential to impede transport of the carriers. Accordingly, the mobility of the carriers in the polycrystals becomes low, making it difficult to obtain a satisfactory on-current. Carriers are generated/re-combined through the trap level at the grain boundary, so that the polysilicon transistor has a high leakage current.
In order to improve electric characteristics of the polysilicon transistor, hydrogenation has been hitherto made. Hydrogen atoms introduced through the hydrogenation are diffused into grain boundaries and bonded to dangling bonds, with the result that the tap density becomes small with a low barrier potential. Accordingly, the mobility of the carriers in the polysilicon transistor becomes high, thereby increasing the on-current. The reduction of the trap level results in the leakage current being suppressed. In addition, part of the introduced hydrogen atoms bonds with an interfacial level at the boundary between the polysilicon and a gate oxide film, thereby causing the threshold voltage of the transistor to be lowered.
In general, hydrogenation is effected by utilizing a passivation film made of silicon nitride. Silicon nitride contains a large amount of hydrogen and serves as a favorable hydrogen source. After formation of a thin film transistor, a silicon nitride protective film is formed and annealed. By the annealing, the hydrogen atoms are diffused into the polysilicon thin film. When the silicon nitride film which has been hydrogenated is left as it is as a passivation layer, there is produced a disadvantage in that the silicon nitride film has a high residual stress, bringing about a significant variation in electric characteristics and particularly a threshold voltage of the thin film transistor. Moreover, when a liquid crystal orienting film such as polyimide is directly formed on the silicon nitride protective film, a disadvantage is involved in that the adhesion between the orienting film and the silicon nitride protective film is not good, resulting in poor uniformity of the orienting film. In addition, when a pair of the substrates are bonded to assemble a liquid crystal cell while leaving the silicon nitride protective film, there is not obtained a satisfactory adhesion strength.
It is accordingly an object of the invention to provide a liquid crystal display device which overcomes the disadvantages of the prior art liquid crystal display devices without use of any SiN protective film for TFT.
It is another object of the invention to provide a liquid crystal display device of the type which has a TFT lower substrate and an upper substrate substantially entirely opposed to each other wherein thin film transistors on the lower substrate are covered with a passivation layer made of an oxide film thereby ensuring stable transistor characteristics.
It is a further object of the invention to provide a liquid crystal display device of the above-mentioned type wherein a passivation layer made of an oxide film and a silicon nitride layer which is selectively formed on thin film transistors and wiring layers is formed whereby the thin film transistor characteristics are prevented from degradation effectively and breakage of the wiring layers is also prevented.
In accordance with one embodiment of the invention, there is provided a liquid crystal display device which comprises:
a first substrate;
a plurality of display elements arranged in a matrix on the first substrate to define a display area of the liquid crystal display device, each of the display elements comprising a picture element electrode and a switching transistor associated with the picture element electrode;
a driving circuit form on the first substrate and connected to the display area, the driving circuit comprising a horizontal driver and a vertical driver;
a second substrate substantially entirely opposed to the first substrate; and
a liquid crystal layer disposed between the first and second substrates.
In accordance with another embodiment of the invention, there is also provided a liquid crystal display device which comprises:
a first substrate;
a plurality of display elements arranged in a matrix on an inner surface of the first substrate, each of the display elements comprising a pixel electrode and a pixel transistor associated with the pixel electrode;
driver means formed directly on the first substrate and connected to the display elements, the driver means including thin film transistors;
a second substrate opposed to the first substrate and having an electrode on an inner surface thereof;
a passivation layer disposed over the pixel transistors and the thin film transistors; and
a liquid crystal layer disposed between the first and second substrates.